[debug] Fix handling of vlas in lto

[debug] Fix handling of vlas in lto

Atm, when running vla-1.c with -O0 -flto, we have:
...
FAIL: gcc.dg/guality/vla-1.c -O0 -flto -fuse-linker-plugin \
  -fno-fat-lto-objects line 17 sizeof (a) == 6
...

The vla a[i + 1] in f1 is gimplified into:
...
f1 (int i)
{
  char a[0:D.1922] [value-expr: *a.0];
  char[0:D.1922] * a.0;

  D.1921 = i + 1;
  D.1926 = (sizetype) D.1921;
  a.0 = __builtin_alloca_with_align (D.1926, 8);
...

The early debug info for the upper bound of the type of vla a that we stream
out is:
...
  DIE    0: DW_TAG_subrange_type (0x7f85029a90f0)
    DW_AT_upper_bound: location descriptor:
      (0x7f85029a9230) DW_OP_GNU_variable_value die -> 0 (0x7f85029a94b0), 0
  DIE    0: DW_TAG_variable (0x7f85029a94b0)
    DW_AT_name: "D.1922"
    DW_AT_type: die -> 0 (0x7f85029a3d70)
    DW_AT_artificial: 1
...

and in ltrans we have for that same upper bound:
...
  DIE    0: DW_TAG_subrange_type (0x7f5183b57d70)
    DW_AT_upper_bound: die -> 0 (0x7f5183b576e0)
  DIE    0: DW_TAG_variable (0x7f5183b576e0)
    DW_AT_name: "D.4278"
    DW_AT_abstract_origin: die -> label: vla_1.c.6719312a + 193 (0x7f5183b57730)
...
where D.4278 has abstract origin D.1922.

The D.4278 die has no DW_AT_location, so when evaluting "sizeof (a)" in the
debugger, we can't find the information to get the value of D.4278, and the
debugger prints "<optimized out>".

This patch fixes that by either:
- adding DW_AT_location to the referenced variable die, or
- instead of using a ref for the upper bound, using an exprloc.

When changing gcc.dg/guality/guality.exp to run the usual flto flavours
"-fno-use-linker-plugin -flto-partition=none" and "-fuse-linker-plugin
-fno-fat-lto-objects" in combination with O0, Og, O1, O2, O3 and Os, this patch
fixes all (20) failures in vla-1.c, leaving only:
...
No symbol "i" in current context.
UNSUPPORTED: gcc.dg/guality/vla-1.c  -O3 -flto -fno-use-linker-plugin \
  -flto-partition=none line 17 i == 5
'a' has unknown type; cast it to its declared type
UNSUPPORTED: gcc.dg/guality/vla-1.c  -O3 -flto -fno-use-linker-plugin \
  -flto-partition=none line 17 sizeof (a) == 6
...

Bootstrapped and reg-tested on x86_64.

2018-08-20  Tom de Vries  <tdevries@suse.de>

	* dwarf2out.c (add_scalar_info): Don't add reference to existing die
	unless the referenced die describes the added property using
	DW_AT_location or DW_AT_const_value.  Fall back to exprloc case.
	Otherwise, add a DW_AT_location to the referenced die.

From-SVN: r263660